1. Field of the Invention
The present invention relates generally to control systems for power trains provided in vehicles, and more particularly, to a system for controlling a power train provided in a vehicle and including an engine having an engine output adjusting device, such as a throttle valve, which is controlled through an electric actuator in accordance with a controlled variable on an accelerator accompanying with the engine and an automatic transmission which is coupled with the engine and in which speed change is automatically performed by means of oil pressure produced in an oil hydraulic control circuit for actuating a speed change mechanism and controlled to vary in accordance with the operating condition of the engine.
2. Description of the Prior Art
There has been proposed a throttle valve control system for controlling electrically a throttle valve in an engine provided in a vehicle to vary its opening degree in accordance with a controlled variable on an accelerator accompanying with the engine, as disclosed in, for example, the Japanese patent application published before examination under the publication number 59/10751. In such a throttle valve control system, a relation between the opening degree of the throttle valve and the controlled variable on the accelerator can be predetermined to be desirable. Therefore, with the throttle valve control system thus proposed previously, it has been put into practice to have a throttle valve operation optimal to the operating condition of the engine by varying a rate of variation in the opening degree of the throttle valve to the controlled variable on the accelerator in accordance with the operating condition of the vehicle engine, for example, in such a manner that the rate of variation in the opening degree of the throttle valve to the controlled variable on the accelerator is increased in response to a requirement of acceleration when the engine is operating with relatively small load or at relatively low speed, or by varying the rate of variation in the opening degree of the throttle valve to the controlled variable on the accelerator in accordance with a rate of change in the controlled variable on the accelerator. Further, it has been also put into practice to vary the predetermined relation between the opening degree of the throttle valve and the controlled variable on the accelerator in response to each of selected speeds in a transmission coupled with the engine to which the throttle valve control system is applied.
Meanwhile, there has been commonly used in vehicles an automatic transmission which is coupled with an engine provided in the vehicle and includes a torque converter, a power transmitting gear arrangement including a speed change mechanism having engaging friction elements, such as clutch and brake elements, operating with oil pressure, and a hydraulic pressure control device including an oil hydraulic control circuit for causing the speed change mechanism to perform speed change. In such an automatic transmission, generally, a speed change control according to a predetermined speed change characteristic is carried out in response to variations in engine load and in travelling speed of the vehicle and an operating oil pressure control in which operating oil pressure in the oil hydraulic control circuit is controlled in response to variations in the engine load so as to have a value appropriate to an engine output transmitted to the speed change mechanism in the power transmitting gear arrangement is also carried out to keep selected one or more of the engaging friction elements to be in engagement without shocks of great degree on the occasion of each speed change. For each of the speed change control and the operating oil pressure control, the engine load is represented by a control variable on an accelerator or an opening degree of a throttle valve.
In the case where the speed change control and the operating oil pressure control proposed previously as mentioned above are performed in the automatic transmission coupled with an engine to which the aforementioned throttle valve control system is applied for controlling electrically a throttle valve in the engine, the following problems arise.
In the engine having the throttle valve controlled electrically and coupled with the automatic transmission, the opening degree of the throttle valve is varied suddenly in response to a small variation in the controlled variable on the accelerator under a condition in which the engine operates with an increased rate of variation in the opening degree of the throttle valve to the controlled variable on the accelerator or the opening degree of the throttle valve is varied in response to a speed change operation in the automatic transmission under a condition in which a relation between the opening degree of the throttle valve and the controlled variable on the accelerator is varied in response to each of selected speeds in the automatic transmission. Therefore, it is feared that an undesirable hunting condition in speed change operation in which unnecessary speed changes are repeatedly carried out is brought about due to the variations in the opening degree of the throttle valve caused in such a manner as mentioned above under the speed change control performed in the automatic transmission. Further, the throttle valve is controlled by an electric actuator to have an opening degree corresponding to the controlled variable on the accelerator and therefore the engine output is not always in a fixed proportion to the controlled variable on the accelerator in the engine having the throttle valve controlled electrically. Therefore, it is also feared that the operating oil pressure in the oil hydraulic control circuit is not controlled to have a pressure value appropriate to the engine output under the operating oil pressure control performed in the automatic transmission.